X-ray densitometer



Nov. 10, 1953 R. BROMBERG ET Al.

X-RAY DENSITOMETER Filed Aug. 7, 19514 mm-...Farm

AT TORNE Y Patented Nov. 10, 1953 fUiNIffflfE-i) .PATENT @OFFICE Angeles, Cali :.,assvignors -to 4vthe United States -ofrimerica Iasijepresented.bythe United States AtomieEnergy; Commission -ApplicationAugust 7, 1951,:Serial No.f240,695

"2 .Claims 1 -The present invention --relates t0 .fail fimproved method -'and=means `for densitymeasuremenband iin moreparticulartofmethod and means iforfcom- .-parative `rdensity `measurements `lhaving :substantiall-yno -eie'ct yon Aor ,fphysical -contact Awith the material iunder test.

tWhile innumerable rdiiferent Ymethods fha-ve fbeen :developed and perfected for 4fdetermining density, the lmajority require .physical :contact with :thefmaterialftoibe yenfieasured or operate :to produce an VYin'terferf'en'ce .-of :fone ,-kind or another -with the fm'aterial :so `fthat :the material fis in :tact aiifected in somerfmanner. Whileithese characteristics `fare =of no :importance in v-many instances, they -d'o in :certain faapplica-tions materially lim-it 4the applicability of lthe method for feven :preclude its inse.

Further wiewing :the zprior sart, i-t `is noted fthat in general :rapid `lvariations .fin density :are mot measurablebyconventional apparatusand-in fact :average density readin'gs i are 'commonly f desired; however, Vvfin fthe instance of high Avelocity iiuid nowandlthe ilke,f-the determination.fofiinstanta neous rdensityiatf a f-Xedpoint'withirespectto said flow viis often of importance.

The jJpres'ent iinvention fis adapted :not `on'l'y Ito measure -transient ldensity `'uetna-'tions .of zhigh, as welllas low, frequencyfbuti-also operates :sub- 'stantially Withouteftectfupon thevmaterial-funder test. "This lis 4`accomplisheddov the Afulfillment lof the `following obi'ectsvfandfin the manner-set :fforth below.

It is an object of 'fthe-present "irn'zention ito 'provide an improved method and means gof meas-urhie-density 'It i is :another Volsi'ject fof fthe present invention to provide .fan improved method fand means fof measurin'gfthefdensity of-aiiiid'inovingat afliigh rateloizfl'o'w.

It lis fane't-her @bj ect 'fof the present inve'ntio'n to provide an 'improvedimethod and fmeansmof measuring :transient -density "Iiiuctuations :having VYhigh or Elovv frequency components.

IEt is still another-fbbject 'o'fthe ipresent invention to yprovidean improved method `:ami :means of 7measuring tra'nsient Sdensity fiiuctua'tions -with alresolution of "theforderf'of :i001 lgram vpep cubic centimeter.

' It is aj-further kobject of fthe present invention to "provide an *improved fniethod `and means for accurately comparing the densities -oi two `fluids or :onefliiid and la solid.

I tis a Vstill further object -of theinvention to provide an vimproved method and meansjforgde'nsity :determinations-by -lX-iray jabsorption.V

Numerous -other Aipossible objects -and adrian Atages #of the invention lWilllbe apparent -fromlthe following description and ,accompanying dramings, wherein:

Figure 1 fis l@schematic representation cifra levice `embodying .the :principles of the present :invention andoperating in conjunction with vasse- `ciatedapparatus; and

Fig. 2 is Yan 'idealized *,volta'ge-tim'e relationship of-the signal ifrom `theradiatiens'detector ofrl'ig.

rConsi-dering now the 'components izand connections 4of an embodiment *lof l`the invention s-ilt notedthatfthe .embodiment is illustratedras operating upon fluid lflow in apparatus for #testi-ng other Huid now characteristics. -5Ihere'1i`s.pro 'videda pressure vessel lfl having internalsthereto Ya thin-Walled tube s|f2 @through which 'water for otheriiuid -Iis 'passed iat highfpressureseandrtem- 1peratures andibywvhichfotlier iiowrneasurements vnot connected withi-the :present invention may be obtained. iFor `the purpose of tliis .invention iza second thin-Walled @tube 1M, Gidentical `to #tube t[12, "is disposed internalito evessel :l hand fadjacntrtube I2. Interior to tube I4 there is provided vWater 'orotherfiuid :I5 in 'a stationart7 .state and at a knownpressure and vtemperature ivvherf'ebythe.I ab'- solute density-'thereof 1^rnaybe accurately deter'- mined. It :is again emphasized 'that ztheaboizee described @apparatus .fdoes not form a necessary part of `tlfiezpresent :invention and is 'only ill-us'- trated as :an example Pof apparatus -with which the present invention may be associatedtfonly being necessary :to provide adjacent fthe :material to be :testedsafstandard ssample lupon whichiabsolute :density measurements ,may be based.

'The presenti invention 'contemplates the smeas= urement of the-comparative absorption of 5 particles :or uva-.ves passed 'through a 'test :sample .'and standard sample .ias fan indication of fthe relative densities-thereof. fIn-ithefillustrated :embodiment the density of Water is tobe measuredfandt-in the test apparatus'illustratedtubesfformediofstainless steel or thelike are employed-'to-fcondu'ot Vthe iiuid flow. `It was determinedstrom `afstudy 'of :the ab.- sorption characteristics of these materials that radiation inthe range l'ot-wave ylengths including X-r'ays Sand y gamnia 'rays is most feasible and 1of thesetwo there is `r'illustr'ated .X-ray irradiation. A suitable -X-ray fbeam Smay -b'e fobtained stroma conventional X-ray source S21 :producing abeam 22 Jwhich lis made substantially monochromatic either at its origin for Joy -suitable #filter-ing.

e The passage v`of 'X-raysJthroughpressure :vessel Il -is facilitated l"by -the provisions -fofiwind'ows 23 and 24 located inthe wall of vessel I-I and d disposed on either side of tubes I2 and I4 and formed of aluminum or other substance relatively transparent to X-rays. Alternate irradiation of tubes I2 and I4 is obtained by the provision of a shutter mechanism disposed in the path oi X- ray beam 22 intermediate X-ray source 2l and tubes I2 and I4, preferably exterior to vessel II. This mechanism may take the form of a chopper wheel 26 having suitably disposed apertures therein and rotating at a fixed speed upon a shaft 2l.

The X-ray beam passing through tubes I2 and I4 is detected by radiation detector 28 disposed on the opposite side of tubes l2 and I4 from X- ray source 2I and an apertured shield 29 limits the detected X-rays to a portion of the beam passing through tubes I2 and I4. Shield 2S which may be disposed either in front or behind tubes I2 and I4 relative to X-ray beam 22, has two apertures therein, one adjacent each tube. Shield 29 is formed of a material that is opaque to X-rays and thus collimates X-ray beam 22 into two small beams which pass one through test tube I2 and one through standard tube I4.

Radiation detector 28 consists in part of luorescing crystals 3I disposed in the path of the X-ray beams that have passed through tubes I2 and I4. The crystal assembly is formed of crystals which emit light in proportion to the amount of X-ray energy passing through tubes I2 and I4 and may be formed of cadmium-tungstate crystals. In View of the difficulty of obtaining a single crystal of suflicient sise, a matrix may be formed by bonding together a plurality of rectangular crystal sections. The light from crystals 3I is directed into a photomultiplier tube 32 either by mounting crystals 3| directly thereon or through the medium of a total reflection light piper 33, as shown, The provision of a light piper is advantageous where it is desired to place the crystals at a distance from photomultiplier tube 32, as for example interior to vessel II, wherein light piper 33 would extend through the wall of vessel II.

The electrical signals from photomultiplier tube 32 are employed to operate a recorder 34 which may be a brush oscillograph and in connection therewith there is provided a band-pass lter 36, an alternating current amplifier 31, a detector 38, and a direct current amplifier 33 through which the signal passes from photomultiplier tube 32 to recorder 34.

The output of radiation detector 28 includes certain signals not useful in measuring density uctuations and resulting from the transient time of the slots in chopper wheel 26. This undesirable signal has a frequency component comparable to that of the idensity fluctuations, as is noted below in more detail, and thus, it is necessary to remove or cancel this signal in order that the desired signal will not be masked. This is accomplished electronically by the provision of a gate signal generator 4I in combination with a driver unit 42 which rotates chopper wheel 2B. Gate signal generator 4I provides a signal in synchronism with the motion of chopper wheel 2E and this signal is amplified by gate signal amplifier 43 and applied to blanking signal generator 44 which in turn produces a signal proportional thereto which is applied to a gate circuit 46 connected between photomultiplier tube 32 and band-pass filter 36. This circuit electronically blanks out undesirable components in the photomultiplier output, as is explained in more detail below.

In addition to the above-noted corrective circuit there is also provided a regulator circuit which compensates for slow variations in characteristics of the apparatus, as for example drift in X-ray intensity, crystal fatigue, and changes in photomultiplier tube characteristics which, if uncorrected, would introduce errors in the measurements. Regulation is accomplished by controlling the gain of photomultiplier tube 32. There is provided a gate circuit 4'I which has its input connected to gate signal amplifier 43 and to photomultiplier tube 32 and which produces a signal proportional to the diierence therebetween and which signal is ampliiied by amplier 48 and applied to the regulator tube in power supply 49 that supplies the dynode voltage for photomultiplier tube 32. By this means the gain of tube 32 is controlled in accordance with slow variations in the output thereof to compensate for such variations.

The foregoing has described the elements of an illustrative embodiment of the invention, and there will now be considered the operation of this embodiment. With water I3 owing through test tube I2 and standar-d tube I4 adjacent thereto lled with stationary water I5 at known temperature and pressure whereby the density thereof may be accurately determined, X-ray source 2l is energized to produce a substantially monochromatic X-ray beam directed upon pressure vessel II about tubes I2 and I4. X-ray beam 22 enters vessel II by means of window 23 and leaves by window 24 in alignment therewith; however, the emergent X-rays are collimated by shield 29 which intercepts all of the X-rays except those passing through a predetermined portion of tube I2 or I4 as determined by the disposition and configuration of the apertures in shield 29. There thus emerges from vessel II two separate X-ray beams of which one has passed through test tube I2 and one through standard tube I4. Shutter mechanism in the form of chopper wheel 2B alternately interrupts portions of the X-ray beam 22 from source 2I so that X-rays pass through tubes I2 and I4 alternately and the X-ray beams emerging from vessel II alternate in time. X-ray absorption in tubes I2 and I4 is a function of the density of the water therein and thus, as tubes I2 and I4 are identical, the relative intensities of the X- ray beams emerging from vessel Ii are proportional to the relative densities of the stationary water I5 in standard tube I4 and flowing water I3 in test tube I2.

The intensity of emergent X-ray beams is measured by radiation detector 28 wherein fluorescing crystals 3|, having the emergent X-ray beams impinging thereon, produce light signals proportional to the intensity of the impinglng X-rays and a light piper 33 transmits these light signals undiminished to photomultiplier tube 32 wherein proportional electrical signals are produced. It will be appreciated that the photomultiplier tube output signal is pulsating direct current with the pulsations being approximately rectangular in shape and the peak to peak amplitude thereof being proportional to difference of X-ray absorption in the two X-ray paths. As the absorption or transmissivity of the paths is a function of density of the water in the tubes I2 and i4 the photomultiplier tube output signal changes with varying density of water in test tube I2 in that the amplitude of pulsations in the signal will be modied in accordance with the density change.

The output signal of pho-l tomultiplier tube 32 is ltered in a, band-pass lter 36 whose band Width is determined .bythe fidelity requirements. ofk the particular apparatus and which serves to cut out a. large proportion of so-called noise signals which. have dinerent frequencies and are inherent to any system such .as that illustrated; and which for example might include signal fluctuationsv produced by v-ray beam variations caused. by a small ripple voltage in the X-ray supply.. The signal from lter 36 is amplied by amplifier 31 and fed into. detector 38 which may include a phase. inverter and which preferably has a small timeY constant. in order not to impair-high` frequencyA response of' the system. A further amplifier 391' may be pro.- vided to. strengthen the signal suiliciently to drive a recorder 3.4 to which itis applied and such amplifier is besty compensated and is most advantageous, when employed with a brush. occillograph recorder as the response thereof rap.- idly decreases with higher frequencies, say about thirty cycles per second. Recorder 34 may, of course, b e any type of indicating instrument suitable for the particular measurements to be made and in this4 respect it is noted that the particular electronic circuit described above is in no way limiting and that various circuits or com'binations of elements may be employed consistent with the requirements of any particular application.

As noted above, the outputA signal from photo- 2 multiplier tube 32 is a pulsating direct current signal with approximately square wave pulsations, as shown in Fig. 2; Actually the signal includes sharpV peaks produced by the chopper wheel action which arer practically unavoidable because of mechanicalv limitations of the chopper wheel. In order to. prevent these' peaks from reaching theA indicating system there is provided a gate;g circuitl 45, intermediate photorm-iltiplierl tube 32, and filter 36. Gate circuit 4E' adds. toV the signal from photomultiplier tube 32 a further signal which may be denominated as a blanking signal and which cancels out certain undesirable portions of the tube signal whereby the signal applied to filter 35 is a substantially square Wave. This blanking signal is derived from the chopper wheel 2E by gate signal generator 4I which might, for example, comprise a light source and photocell. The signal from gate signal generator 4I may be amplified, as by gate signal amplier 43, and fed into blanking signal generator 44 which produces a signal in synchronism therewith that is applied to control gate 'circuit 46. By this means compensation is provided for undesirable frequency components in the photomultiplier tube output.

' The drift compensating regulator system noted above is controlled in part by the amplied signal from gate signal generator 4I which is applied to gate circuit 41. Also applied to an input circuit of gate circuit 41 is a portion of the photomultiplier tube output signal. As the signal from gate signal generator 4I depends upon the angular position of chopper wheel 26, it is employed to control the gating action of gate circuit 41 so that this circuit passes only the portion of photomultiplier tube output that is derived Ifrom the X-rays passing through standard tube I4. This portion of photomultiplier tube output is applied by gate circuit 41 to regulator amplifier 48 which provides an output dependent upon the difference between the input signal and a constant voltage. This regulator amplier output is applied to control power supply 49 which energizes the dynodes of photomultiplier tube 32. By this means the sensitivity oiphot'omultiplier tube 32v is controlledk or regulated to maintain constant the photom'ul-- tiplier tube signal produced by X-rays `passing through standard tube I4v and thus to electron'- ically compensate for slow variations as result? crystal fatigu,

meter. It is noted that X-ray absorption issn'l exponential function; however, only about- 10% of the X-rays passing through the tubes l2' and, I4 are absorbed and thus the absorption el'ect;l

lies within the rst 10% of an exponential curve"`V and therefore a substantially linear relationship is obtained. While the use7 of alinear relationey ship is highly desirable it isnecessary in the pres'- ent instance to measure the X-ray beam with terr times the desired', accuracy of density measurement as only about 10% of the X-rays are absorbed'; This is accomplished by operating" the photomultiplier input at a high level which reduces the proportionate effect of the dark cin-k rent noise and by measuring only density dif? ference's. By employirigwA. C; amplifier' 31v and' lter 36 only theYV variations in signal arev passed` on to recorder 34, andnoise'of other frequencies'.

and the" direct current base are excluded. Thus,

atY recorder 36 there' is indicated' the relativev densities of flowing water I 3 and stationary water I5, and absolute density measurements of ilowing water I3 are determined from the application of this ratio to the known and constant density of stationary water I5 in standard tube I4.

The present invention has been disclosed with respect to but a single preferred embodiment; however, it will be apparent to those skilled in the art that numerous variations and modifications are possible within the spirit and scope of the invention and thus the invention is not to be limited except as defined in the following claims.

What is claimed is: I

1. An improved comparative densitometer for indicating the standard relative density of a pair of objects and comprising an X-ray source emitting a substantially mono-energetic beam of X-rays directed upon said objects, an interrupter disposed intermediate said source and said objects and alternatively shielding each of said objects from said source, a radiation detector disposed on the opposite side of said objects from said source and in aligrnnent therewith, said radiation detector having controlled amplification means and an output, a first gate circuit having an input and an output with the input thereof connected to the output of said radiation detector, indicating means connected to the output of said gate circuit, a gate signal generator energized by said interrupter to produce signals in synchronism with the alternating X-ray beam striking one of said objects, a blanking signal generator connected to and controlled by said gate signal generator and producing a blanking tube character the present vention measures density fluctuation frequency:` components from 0 to 100 cycles per second' and with a resolution of 0.01 gram per cubic centi= i signal. said iirst gate circuit having control means energized by said blanking signal to protect said indicating device from undesirable signals of said radiation detector, power supply means including a standard voltage source and having a variable voltage output applied to the control means of said radiation detector, and a second gate circuit having an input connected to the output of said radiation detector and an output connected to said power supply means, said second gate circuit having a gate control connected to said gate signal generator whereby said radiation detector is controlled to produce at the output thereof a signal of constant amplitude for incident X-rays transmitted through one of said objects.

2. A comparative densitometer for indicating the instantaneous relative densities of a pair of objects comprising in combination an X-ray source producing an X-ray beam of substantially monochromatic energy directed upon said objects, a radiation detector disposed on the opposite side of said objects from said X-ray source and including fluorescing crystals having the X-rays transmitted through said objects impinging thereon and a photomultiplier tube adjacent said crystals for producing electrical signals proportional to the intensity of X-rays impinging upon said crystals, shutter mechanism intermediate said X-ray source and said objects and alternately shielding each of said objects whereby said objects are alternately irradiated, a power supply having control means and energizing said photomultiplier tube to operate at high level output, a gate circuit having control means and an input and output circuit, said input circuit being connected to the output of said photomultiplier tube, a gate signal generator connected to said shutter mechanism and producing a gate signal in synchronism therewith, means impressing said gate signal upon the control means of said gate circuit to render said gate circuit con- Cir ducting during the periods said shutter mechanism is passing X-rays toward a predetermined one of said objects, a regulator-amplier connected to the output of said gate circuit and receiving electrical signals from said photomultiplier tube produced by X-rays passing through the predetermined one of said objects, said regulator-amplier having a standard voltage source and comparing the received signals thereto to produce a difference voltage, means impressing said difference voltage upon the control means of said photomultiplier tube power supply Whereby the photomultiplier tube output signals produced by X-rays passing through the predetermined one of said objects is regulated, a band pass filter filtering the output of said photomultiplier tube and passing only signals of the same frequency as the alternation of said X-ray beam, an alternating current amplier connected to said band pass filter, and indicating means connected to said amplifier and indicating the signals passed therethrough as a measure of the relative X-ray transmissivity of said objects and thus of the density thereof.

ROBERT BROMBERG. WILLIAM L. MARTIN.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,488,269 Clapp Nov. 15, 1949 2,539,203 Pohl Jan. 23, 1951 2,583,132 Altar et al Jan. 22, 1952 OTHER REFERENCES X-Ray Photometer, Michel et al., General Electric Review, February 1947, pages 45-48.

X-Ray Thickness Gauge for Cold-Rolled Strip- Steel, Lundahl, A. I. E. E. Transactions, 1948, vol.

40 67, pages 83-90. 

